This application relates to detection apparatus for security and surveillance systems, in particular but not necessarily exclusively for systems relying on magnetic detection of special markers or tags, which are often used in electronic article surveillance (EAS), e.g. in retail premises.
Detection systems in general use large, relatively flat, pile-wound, air-cored induction coils for reception of ac magnetic fields generated when tags pass through the detection zone. The coil axis is usually perpendicular to the direction of travel of persons walking through the detection zone, This type of detection system is prone to interference from external sources of ac magnetic fields such as cash registers, motors and electrical cables, since these will also induce voltages in the pick-up coils. These extraneous signals complicate the recognition of the signals from the markers, and generally cause false alarms or reduce the genuine detection rate. Additionally, this type of detection suffers from further unwanted signals which are generated by external (normally) `passive` objects such as iron and steel panels or other metal fixtures close to the detection volume, since these objects are driven to produce unwanted magnetic: signals by the magnetic field which is generated by the EAS system, which is used to interrogate the tags in and around the detection volume.
Screen material can be employed to shield the air-cored detection coils from unwanted external signals, but these have to cover at least the entire area of the coil, so are expensive, cumbersome, difficult to install and aesthetically undesirable.